The present invention relates to a drawing mechanism for a press equipped with a mechanically actuated drawing ram and with a sheet metal holder, with pressure cylinders for holding the sheet metal part during the drawing operation and with at least one pressure cylinder for the ejection of the workpiece and with a pressure plate adapted to be moved in a press table by the piston rods of the pressure cylinders and transmitting the movement and forces of the pressure cylinders by way of pressure rods to the sheet metal holder.
In order to prevent that the pressure plate is a press equipped with a drawing mechanism follows directly the drawing ram after the deformation, different pressure cylinder systems are provided for the ejection of the workpiece and the retention of the workpiece during the drawing which engage in different areas at the pressure plate.
Such a drawing installation is described in the DE No. 35 05 984. The different individual functions of the drawing mechanism such as control of the sheet metal holder pressure, control of the sheet metal holder upward movement, control of the ejector movement and of the end position abutment are adjustable and controllable independently of one another. The drawing installation includes pressure cylinders for the sheet metal retention during the drawing operation and pressure cylinders for the ejection of the workpiece as well as a pressure plate guided in the press table and movable and supported by the pressure cylinders. After the working stroke of the drawing ram, follow-up control systems become operable for an upward movement, independent in time, of the ejector- and drawing-piston rods. The drawing piston rods are disengageable for that purpose from the bottom side of the pressure plate.
During the working stroke of the drawing ram of the press, at first the sheet metal part inserted into the work tool is pressed by the drawing ring against the sheet metal holder and thereafter is displaced together with the sheet metal holder against the holding force produced by the drawing installation. In addition to the holding force to be supplied by the drawing mechanism, the drawing ram must also overcome the inertia forces which become effective at the beginning of the deformation as a result of the acceleration of the movable masses of the sheet metal holder and of the drawing mechanism.
For avoiding this load, which occurs impact-like on the drawing ram, it is known in connection with the drawing installation according to the EP No. 0 074 421 A1 to displace the several pressure rods for the pressure plate carrying the sheet metal holder by way of a piston rod, adapted to be actuated by a pressure medium, of a pressure cylinder fixed in the press. The piston rod is operatively connected with a pressure piston which has an operating surface adapted to be acted upon in the drawing direction. The operating surface is adapted to be acted upon directly from a pressure reservoir by way of a control system operating in dependence on ram travel so that the sheet metal holder is adapted to be pre-accelerated in the drawing direction prior to the touchdown of the drawing ram on the sheet metal holder. The movement of the piston rod of the drawing mechanism is controlled in time during the pre-acceleration phase and provides no indication of a pre-acceleration of separately arranged and operating drawing and ejector pistons.
In contrast thereto, it is the object of the present invention to control the pre-acceleration of the piston rods for the sheet metal holder pressure and the ejection operation in the drawing direction by way of deliberately predetermined metered quantities in hydraulic liquid and to control the refill in hydraulic liquid from a reservoir during the further movement of the piston rods in the drawing direction.
The underlying problems are solved according to the present invention in that each piston rod includes a pressure piston with an operating surface adapted to be acted upon in the drawing direction, in that each of the pressure spaces coordinated to an operating surface is in fluid communication, on the one hand, with a pressure quantity space of a pre-acceleration cylinder consisting of pressure quantity space, pressure space and a pressure piston separating these spaces from one another, and in that the, respectively, each pressure space is adapted to be pressure-actuated by a control system operating as a function of ram travel by way of a throttling directional control valve, whereby prior to the contact of the drawing ram on the sheet metal holder, the latter is adapted to be preaccelerated in the drawing direction by actuation of the operating surfaces with hydraulic liquid from the pressure quantity spaces.
Of particular advantage is thereby the quantitative control of the pre-acceleration of the piston rods in the drawing direction and the pre-control adapted to be influenced by way of a directional control valve variable in cross section by way of a pre-acceleration cylinder. The synchronous pre-acceleration is assured by the separation of the pressure spaces from one another. Pressure vibrations between the sheet metal part and the sheet metal holder are precluded. The sheet metal part is retained uniformly already during the beginning of the drawing phase.